A variety of processing methods are used to process polymeric materials, especially polymeric olefinic materials, to prepare articles. Many of these processing methods involve the use of heat, pressure or a combination thereof. Examples of processing methods used include, for example, hot melt extrusion and injection molding. Among the articles prepared by hot melt extrusion techniques are polymeric webs. Examples of useful polymeric webs include films and non-woven fibrous webs. Among the uses for such polymeric webs are electret articles.
An electret is a dielectric material that exhibits a quasi-permanent electrical charge. Electrets are useful in a variety of devices including, e.g. cling films, air filters, filtering facepieces, and respirators, and as electrostatic elements in electro-acoustic devices such as microphones, headphones, and electrostatic recorders.
The performance of microfibrous webs used for aerosol filtration can be improved by imparting an electrical charge to the fibers, forming an electret material. In particular, electrets are effective in enhancing particle capture in aerosol filters. A number of methods are known for forming electret materials in microfibrous webs. Such methods include, for example, bombarding melt-blown fibers as they issue from the die orifices, as the fibers are formed, with electrically charged particles such as electrons or ions. Other methods include, for example, charging the fibers after the web is formed, by means of a DC corona discharge or imparting a charge to the fiber mat by means of carding and/or needle tacking (tribocharging). Recently, a method in which jets of water or a stream of water droplets impinge on a non-woven web at a pressure sufficient to provide filtration enhancing electret charge has been described (hydrocharging).